Mastication is a complex sensory-motor behavior necessary for the preparation of food for swallowing. The movement of the jaw plays a critical role in the breaking down of food. Abnormal jaw movements are manifested in several neurological disorders such as bruxism, temporomandibular joint disorders, and oromandibular dystonia. Thus, understanding the neural control of masticatory movements is essential for future development of effective and targeted treatments of these diseases. It is known that trigeminal motor neurons (MoVs) provide the main motor control of jaw muscles. However, the pre-motor circuits that provide direct inputs onto MoVs and thereby control their activity remain poorly understood. Furthermore, tonic stimulation applied to a sub-region of the jaw motor cortex deemed the cortical masticatory area (CMA) can activate rhythmic jaw movements. Yet the motor cortex is not known to form a direct connection with MoVs. The key groups of pre-motor neurons that relay cortical commands to MoVs have not been determined. In preliminary studies, a monosynaptic rabies virus based transsynaptic tracing technique was used to find that neurons located in the lateral paragigantocellular reticular nucleus (LPGi) are directly connected to MoVs innervating the main jaw closing muscle, the masseter, and that the number of LPGi premotor neurons that synapse onto the masseter MoVs dramatically increases with the transition from suckling to chewing behavior. These findings, together with previous studies, lead me to propose a central hypothesis: MoV pre-motor neurons in the LPGi form a critical neural module to relay cortical signals both directly onto jaw closing MoVs and onto other pre-motor neurons in the brainstem reticular network to enable rhythmic and tonic jaw movements. I will use optogenetic-assisted slice electrophysiology to determine functional connectivity between LPGi pre-motor neurons and other proposed neuronal groups. I will use optogenetic activation and inhibition to determine the role of LPGi pre-motor neurons in cortically-induced fictive rhythmic jaw movements in anesthetized mice. Finally, I will examine the consequences of silencing LPGi pre-motor neurons in natural mastication in awake behaving mice. These studies are expected to provide much needed novel insights into the precise neural circuitry controlling jaw movement and masticatory behavior.

Public Health Relevance

Neurological disorders such as bruxism, temporomandibular joint disorders, and oromandibular dystonia all share a common symptom of abnormal or difficult jaw movements. I will use a combination of anatomical, electrophysiological, optogenetic and behavioral analyses to dissect the functions of a key group of brainstem pre-motor neurons in controlling rhythmic and tonic jaw movements. This work will help identify a precise neural target for developing future treatments of jaw movement disorders.